Method for Making Corrosion Resistant Fluid Conducting Parts

ABSTRACT

A method for making a tube is described in which a multi-layer billet is extruded to provide a tube having a wall comprising an inner layer metallurgically bonded to an outer layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation application and claims thebenefit of the filing date under 35 U.S.C. §120 of co-pending U.S.patent application Ser. No. 12/839,618, filed on Jul. 20, 2010, which isa continuation application and claims the benefit of the filing dateunder 35 U.S.C. §120 of U.S. patent application Ser. No. 11/061,355,filed on Feb. 18, 2005, now U.S. Pat. No. 7,922,065 B2, which claimspriority under 35 U.S.C. §119(e) to U.S. Provisional Patent ApplicationNo. 60/598,228, filed on Aug. 2, 2004. U.S. patent application Ser. Nos.12/839,618; 11/061,355; and 60/598,228 are incorporated-by-referenceinto this specification.

TECHNICAL FIELD

The present disclosure is directed to corrosion resistant fluidconducting parts, methods of making such parts, and equipment includingone or more such parts. The present disclosure also is directed tomethods of replacing one or more fluid conducting parts of an article ofequipment with improved, corrosion resistant fluid conducting parts. Thepresent disclosure is further directed to articles from which corrosionresistant fluid conducting parts can be formed.

BACKGROUND

Various industrial processes and equipment operate at very highpressures and temperatures. For example, throughout the world theindustrial scale process for synthesizing urea involves the reaction ofammonia and carbon dioxide in large high-pressure reactors attemperatures in excess of 150° C. (302° F.) and pressures ofapproximately 150 bar (15.0 MPa). The process is well known and isdescribed in, for example, U.S. Pat. Nos. 4,210,600, 4,899,813,6,010,669, and 6,412,684. In the process, ammonia, which is generally inexcess, and carbon dioxide are reacted in one or more reactors,obtaining as end products an aqueous solution containing urea, ammoniumcarbamate not transformed into urea, and the excess ammonia used in thesynthesis.

The most corrosive conditions during urea synthesis occur when theammonium carbamate is at its highest concentration and temperature.Although these conditions occur at the most critical step in theprocess, only relatively few materials can withstand the conditionswithout experiencing significant corrosion, which can lead to equipmentfailure. Materials from which urea synthesis equipment has beenfabricated have included in part, over time, AISI 316L stainless steel,INOX 25/22/2 Cr/Ni/Mo stainless steel, lead, titanium, Safurex®stainless steel, and zirconium.

When the urea synthesis process was first developed, “urea grade”austenite-ferrite stainless steels and other proprietary grades ofstainless steel were used. The synthesis equipment includes a stripperhaving a vertical tube bundle in which the urea process medium isdecomposed and condensed. The urea process medium flows through theinner volume of the tubes, while saturated steam circulates andcondenses on the outside of the tubes. The condensing steam provides thenecessary energy to decompose the excess ammonia and ammonium carbamatewithin the tubes into urea and water. The spacing of the tubes in thestripper is maintained by tubesheets, which include circular holesthrough which the tubes pass, and the individual tubes also are joinedto a surface of the tubesheets by strength welds.

Few materials can withstand the internal and external conditions towhich the stripper tubes are subjected without experiencing significantcorrosion and/or erosion over time. The corrosion resistance ofstainless steels used in stripper tubes is largely dependent on whetherthe urea solution in the tubes is uniformly and evenly distributed onthe tube surfaces so as to passivate the stainless steel (the solutionprovides a portion of the passivating oxygen). If the tubes' internalsurfaces are not fully and continuously wetted, the stainless steel willcorrode. Thus, if the processing unit is operated at a steady-statecondition and at relatively high capacity, the stainless steel tubeswill perform adequately. If the unit is operated at lower capacity,however, distribution of the urea process medium in the stripper tubesmay be uneven or the tubes may include unwetted internal surfaces thatare not totally passivated, resulting in corrosion. Thus, currentlyavailable stainless steels were not found to be reliable stripper tubematerials for use in the urea synthesis process.

To address the corrosion problems experienced with stainless steels,over 30 years ago urea synthesis equipment fabricated from titanium wasdeveloped. In this design, the titanium-clad stripper includes solidtitanium tubes joined to titanium-clad tubesheets. When this design wasplaced in service, the vertically disposed stripper tubes were subjectto corrosion and erosion in a region in the vicinity of the strengthwelds fusing the tubes to the stripper tubesheets. Erosion and corrosionwere also noted within the first 1 meter (39.4 inches) length of thetubes. The ammonium carbamate is at the highest concentration andtemperature, and decomposes and condenses in this region, and it ispostulated that the erosion/corrosion occurs because of the suddenchange in fluid direction, fluid impingement, or sudden evaporation inthis region. After the propensity for corrosion/erosion of titaniumstripper tubes was identified, the equipment was redesigned so that thestripper units could be flipped end-to-end, thereby allowing forerosion/corrosion to occur on both ends of the stripper tubes beforereplacement of the tubes was necessary. Although this almost doubled theservice life of the stripper tubes, it was not a permanent solution tothe units' corrosion problem, and many of the urea processing unitsfabricated with titanium stripper tubes have experienced some degree oferosion/corrosion problems.

To further address the erosion and corrosion problems experienced inurea strippers, stripper tubes fabricated using zirconium wereintroduced, as described in U.S. Pat. No. 4,899,813. Because zirconiumis more expensive than titanium and stainless steel, earlyzirconium-equipped stripper tubes were designed to include a stainlesssteel outer tube (generally 2 mm (0.8 inch) minimum thickness) and arelatively thin tubular inner liner of zirconium (generally 0.7 mm (0.03inch) minimum thickness) mechanically bonded (snug fit) within thestainless steel tube. The mechanical bonding necessary to retain thezirconium liner in place was achieved by expanding the inner diameter ofthe zirconium liner so as to snugly fit within the stainless steel outertube. The stainless steel outer tube of the resulting snug fitdual-layer tubing provides mechanical strength and also reduced thecosts of the tubing relative to solid zirconium tubing. The relativelythin zirconium liner provides improved corrosion resistance. Zirconiumwas selected for this application because it exhibits excellentcorrosion resistance in highly corrosive, high pressure, hightemperature environments.

The foregoing stainless steel/zirconium snug fit dual-layer strippertubing was manufactured under stringent requirements to better insure avery tight mechanical fit. Nevertheless, the mechanical bonding of thelayers proved to be a source of trouble in tubes intended for longservice lifetimes. Because of the absence of a metallurgical bondbetween the corrosion resistant zirconium liner and the stainless steelouter tube, a slight gap existed between the zirconium inner liner andthe stainless steel outer tube. This gap, in part, resulted from thedifferent mechanical and physical properties of zirconium and stainlesssteels. For example, the materials have very different thermal expansioncoefficients and, when heated, stainless steel will expand to a greaterdegree than zirconium. Also, because of the dissimilar properties of thematerials, they cannot be fusion welded together, and it becamenecessary to remove a portion of the zirconium liner from the strippertube end in order to fusion weld the tube to the stainless steeltubesheets. Regardless of how well the stainless steel tubes andzirconium liners were fabricated and how tightly the tube componentswere mechanically fit together, it was found that over time corrosiveurea process medium was able to infiltrate the small gap between thestainless steel and the zirconium, resulting in crevice corrosion and,finally, penetration of the stainless steel outer tube. In some ureastrippers having this design, the tubes began to fail for this reason,requiring shutdown of the urea synthesis equipment to repair theproblem, and resulting in substantial maintenance costs.

Yet another, recent development is a design for urea synthesis strippertube bundles including solid zirconium stripper tubes, zirconium-cladtubesheets, and an explosive bonded zirconium cladding layer on allinternal wetted surfaces. However, given the cost of urea synthesisequipment, it is typically less expensive to repair corroded parts ofexisting equipment than to replace the equipment with this new corrosionresistant design. While parts replacement may be a cost-effective optionfor stripper equipment including solid zirconium stripper tubes,zirconium-clad tubesheets, and zirconium cladding on wetted surfaces, itwould be advantageous if titanium clad stripper units could bemanufactured with stripper tubes having improved corrosion resistance.That is because titanium-clad stripper units tend to be significantlyless expensive to manufacture than zirconium-clad units.

Accordingly, it would be advantageous to provide an improved design forstripper tubes of urea synthesis equipment. It also would beadvantageous to provide a method of retrofitting existing strippers forurea synthesis equipment with a form of corrosion resistant replacementstripper tubes, while utilizing the strippers' existing tubesheets.

More generally, it would be advantageous to provide an improved designfor and method for producing corrosion resistant fluid conducting partsfor articles of equipment operating under conditions promotingcorrosion. In addition to stripper units of urea synthesis equipment,such articles of equipment include, for example, other chemicalprocessing equipment, condenser units, and heat exchanger equipment. Italso would be advantageous to provide a method of retrofitting existingworn and/or corrosion-prone parts of equipment with corrosion resistantreplacement parts, wherein the replacement parts are fabricated fromcorrosion resistant materials such as, for example, zirconium, zirconiumalloys, titanium, titanium alloys, and stainless steels.

SUMMARY

A method for making a tube comprises providing hollow first and secondcylinders. The hollow first cylinder comprises zirconium, a zirconiumalloy, hafnium, a hafnium alloy, vanadium, a vanadium alloy, niobium, aniobium alloy, tantalum, or a tantalum alloy. The hollow second cylindercomprises stainless steel, titanium, or a titanium alloy. The firstcylinder is positioned within the second cylinder so that the outersurface of the first cylinder opposes the inner surface of the secondcylinder. The end joints between the first cylinder and the secondcylinder are welded together to provide a multi-layer billet. Themulti-layer billet is heated to an extrusion temperature. The heatedmulti-layer billet is extruded to metallurgically bond the outer surfaceof the first cylinder to the inner surface of the second cylinder andprovide a tube having a wall comprising an inner layer metallurgicallybonded to an outer layer. The tube can be cold worked to reduce the wallthickness and/or diameter of the tube.

The reader will appreciate the foregoing details and advantages, as wellas others, upon consideration of the following detailed description ofcertain non-limiting embodiments of the methods, articles and parts ofthe present disclosure. The reader also may comprehend such additionaladvantages and details upon carrying out or using the methods, articles,and parts described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the methods may be better understood byreference to the accompanying drawings in which:

FIG. 1 illustrates one embodiment of a stripper tube according to thepresent disclosure, wherein the tube includes a first fluid conductingregion, fabricated from zirconium, and joined by inertia welding oranother solid state welding technique to a second fluid conductingregion, fabricated from titanium.

FIG. 2 illustrates an arrangement for mounting the stripper tube of FIG.1 to a titanium-clad surface of a stripper tubesheet and which includesthe use of a multi-layer fluid conducting tube end.

FIG. 3 schematically illustrates an embodiment of a process forfabricating a multi-layer fluid conducting part according to the presentdisclosure.

FIG. 4 schematically illustrates an end of a welded bi-layer billet madeas an intermediate article in the process of FIG. 3.

FIG. 5 illustrates an arrangement for mounting an embodiment of strippertube including a multi-layer tube end according to the presentdisclosure to a titanium-clad surface of a stripper tubesheet.

FIG. 6 depicts unsectioned and sectioned samples of a zirconium tubesection that has been inertia welded to a titanium tube sectionaccording to an aspect of the present disclosure.

FIG. 7 depicts two samples of a zirconium tube section inertia welded toa titanium tube section according to an aspect of the presentdisclosure, and wherein the resultant zirconium/titanium fluidconducting tube has been machined to remove flash.

FIG. 8 is a photograph of a cross-section of a zirconium to titaniumweld interface in the tube wall of an inertia welded sample according toan aspect of the present disclosure.

[FIG. 9 is a high magnification view of the weld interface shown in FIG.8.

FIG. 10 is a high magnification image of a portion of the weld interfaceregion shown in FIG. 9.

FIGS. 11 and 12 are schematic representations of steps of an embodimentof a process according to the present disclosure for fabricating amulti-layer fluid conducting part or part section.

FIG. 13 illustrates an end view of a welded bi-layer billet made as anintermediate article in one of the process steps included in FIG. 12.

FIG. 14 is a photomicrograph of the metallurgical bond region of a heattreated multi-layer tube made by an embodiment of a method according tothe present disclosure.

DETAILED DESCRIPTION

Certain non-limiting embodiments provided in the present disclosureinclude novel corrosion resistant fluid conducting parts, equipmentincluding one or more such parts, and methods for replacing fluidconducting parts of equipment subject to corrosive and/or erosiveconditions with corrosion resistant fluid conducting replacement parts.Examples of the fluid include a gas, a liquid, or a gas/liquid mixture.Non-limiting embodiments of the novel parts include, for example, partshaving cylindrical or other shapes, tubes, pipes, nozzles, stub ends,tube connectors, pipe connectors, stripper tubes, heat exchanger tubes,and other fluid conducting parts.

Certain non-limiting embodiments of the fluid conducting parts includeat least one first fluid conducting region fabricated from at least onecorrosion resistant material such as, for example, zirconium, titanium,tantalum, niobium, alloys of any of these metals, or another corrosionresistant metal or alloy. The parts also include at least one secondfluid conducting region including a material that is compositionallyidentical or compositionally substantially identical to the materialfrom which an existing mounting region of the equipment to which thepart is to be mounted is formed. The corrosion resistant first region isdirectly or indirectly joined to the second region by solid statewelding to form a unitary fluid conducting part such as, for example, atube or a pipe. Such part may be secured to an article of equipment bywelding together the identical or substantially identical materials ofthe second region and the mounting part of the equipment. The identicalor substantially identical materials may be fusion welded, such as by,for example, autogenous welding or use of a welding filler metal,without generating conditions in the vicinity of the fusion weld thatwill significantly promote corrosion.

The parts and methods described in the present disclosure may be adaptedfor use with various types of chemical processing and other equipment.Non-limiting embodiments of such equipment and the particular fluidconducting parts of such equipment that may be constructed according tothe present disclosure include tubing for urea strippers, carbamatecondensers, and bi-metallic strippers, and heat exchanger tubing andpipes for chemical and petrochemical processes.

A particular non-limiting embodiment described herein is a method ofreplacing corroded and/or eroded titanium stripper tubes in ureasynthesis equipment with replacement tubes comprising a corrosionresistant metal or alloy region, such as a zirconium or zirconium alloyregion, which would be highly resistant to the corrosive/erosive effectsof the urea process media within the tubes. The method allows astripper's existing titanium-clad tubesheets and exchanger heads to bereused, so it is not necessary to replace the entire stripper unit. Themethod involves providing replacement stripper tubes having (i) atubular corrosion resistant region fabricated from, for example,zirconium or a corrosion resistant zirconium alloy, and (ii) at leastone tubular mounting region fabricated from, for example, titanium oranother metal or alloy that may be fusion welded to the titanium-cladtubesheet of the stripper without generating conditions in the vicinityof the fusion weld that significantly promote corrosion or erosion. Thecorrosion resistant region and the mounting region are joined eitherdirectly or indirectly by a solid state welding technique to form thefluid conducting replacement part.

FIG. 1 is a sectioned view of one non-limiting embodiment of a strippertube 10 constructed according to the present disclosure. The tube 10,for example, may be provided as an original part of a stripper unit or,as discussed above, may be used as a replacement stripper tube toretrofit an existing stripper unit. The stripper tube 10 includes acylindrical passage 12 defined by continuous wall 13. A central portionof the continuous wall 13 of tube 10 is a corrosion resistant zirconiumtube 14. A length of titanium tubing 16 is inertia welded on each end ofthe zirconium tube 14. The titanium tube ends 16 may be fusion welded toan existing titanium-clad tubesheet in the stripper unit withoutproducing a dissimilar zirconium-to-titanium fusion weld. FIG. 2 showsone possible arrangement for a tube-to-tubesheet weld to secure strippertube 10 through a tube hole in a tubesheet 20. It will be understoodthat the mounting configuration shown in FIG. 2 may be used wheninitially manufacturing a stripper, or may be used when replacingstripper tubes in an existing stripper that is in service. Tube 10,which includes titanium tube end 16 inertia welded at region 17 tozirconium tube region 14, is disposed through a bore in the titaniumcladder sheet 24 of the tubesheet 20. Regions 26 are carbon steel orstainless steel regions of the tubesheet 20. Tube 10 is secured totubesheet 20 by a titanium strength weld 28 at a junction of thetitanium tube end 16 and the titanium cladder sheet 24. Thus, the fusionweld region is entirely of titanium, and no alloys combining titaniumand zirconium are generated in the fusion weld region.

As discussed below, it is believed that alloys formed in the weld regionwhen fusion welding dissimilar metals, such as the zirconium-titaniumalloys formed when fusion welding together zirconium and titanium, havea propensity to corrode when subjected to corrosive substances and/orconditions. Solid state welding, however, does not generate alloys inany significant amounts. Accordingly, by providing fluid conductingparts having a highly corrosion resistant region solid state welded to aregion including material that is compositionally identical orcompositionally substantially identical to a mounting part of theequipment, or that otherwise does not produce alloys prone to corrosionwhen fusion welded to the mounting part, the present method allowsequipment to be manufactured or retrofitted with corrosion resistantparts without creating conditions promoting corrosion.

As used herein, solid state welding refers to a group of weldingprocesses that produce coalescence at temperatures essentially below themelting point of the base materials being joined, without the additionof brazing filler metal. Pressure may or may not be used during varioussolid state welding processes. Non-limiting examples of solid statewelding techniques that may be used in embodiments of the methodsdisclosed herein include, for example, cold welding, diffusion welding,explosion welding, forge welding, friction welding (including inertiawelding), hot pressure welding, roll welding, and ultrasonic welding.These techniques have been used for many years in other applications andare well known to those having ordinary skill. As such, an extendeddiscussion of such joining techniques need not be presented herein toallow those of ordinary skill to practice the present methods.

Solid state welding fundamentally differs from fusion welding, whereinthe materials to be joined are melted during the joining process. In thecase where the fusion welded materials are not identical, the fusionweld region necessarily includes alloys of the joined materials. Fusionwelding zirconium directly to titanium, for example, would create alloysthat enhance corrosion/erosion rates in the vicinity of the weld region.Fusion welding of zirconium and titanium also will cause solid solutionhardening in the resultant weld, which, in turn, reduces weld ductilityand significantly increases weld hardness. The resultant alloy mixacross the zirconium to titanium weld joint includes a range ofzirconium-titanium alloy mixtures (of 100% titanium to 100% zirconium,and all combinations in between). The alloy compositions found in adissimilar zirconium to titanium weld will have different mechanicalproperties and corrosion properties, which are impossible to accuratelycontrol during the welding process. Mechanically, the alloys ofzirconium and titanium are very high strength and can have very highhardness, which can be up to twice as hard as either of the pure metalsalone. Other mechanical properties that may be affected by fusionwelding are notch sensitivity and formability. Thus, certain regions ofa zirconium/titanium fusion weld may exhibit mechanical properties thatare not acceptable if substantial pressures are generated within theequipment. Certain alloy compositions (regions of the weld mixture) willexperience very high oxidation and corrosion rates.

Generally, the resultant corrosion resistance of a metal welded to adissimilar metal will have a much lower corrosion resistance than thatof either pure metal alone, and that is the case in the fusion weldingof zirconium and titanium. Even if a pure zirconium or titanium fillermetal is used, there will be an area in the weld in which azirconium-titanium alloy exists having low corrosion resistance relativeto either pure metal alone. A Huey corrosion test is a standardcorrosion screening test for materials used in applications in which thematerials contact nitric acid and/or urea. It has been determined thatin a Huey corrosion test, for example, a zirconium-titanium fusion weldwill exhibit a high rate of corrosion, while a titanium-titanium orzirconium-zirconium weld will exhibit a very low corrosion rate.

Thus, by solid state welding the zirconium and titanium fluid conductingregions together and fusion welding one or more of the titanium tuberegions to the titanium cladding of the tubesheet, the foregoingnon-limiting embodiment described herein avoids fusion welding ofdissimilar materials. This, in turn, avoids producing alloys within weldregions having relatively high corrosion/erosion rates when exposed tothe urea process media and other corrosion promoting conditions withinthe stripper of urea synthesis equipment. A significant enhancement inservice life of the newly manufactured or retrofitted stripper shouldresult.

Given its reproducibility and ready adaptation to fuse tubular andcylindrical members, inertia welding may be readily applied to formembodiments of the novel parts described herein. As is known in the art,inertia welding is a solid state welding technique that is a type offriction welding wherein the materials to be joined are forged togetherwithout melting the materials. In inertia welding the energy required tomake the weld is supplied primarily by the stored rotational kineticenergy of the welding machine. One of the two workpieces is held on arotatable spindle attached to a flywheel of a specific mass. The otherworkpiece is held in a chucking device and is restrained from rotating.The flywheel is accelerated to a predetermined rotational speed and thendisengaged, so that the rotating components are free to rotate with aspecific kinetic energy. At the time the flywheel drive motor is turnedoff, the workpieces are forced together with an axially appliedpressure, which in some techniques may be increased during the weldcycle. The kinetic energy stored in the rotating flywheel is dissipatedas heat through friction between the workpieces at the weld interface,and this large localized energy bonds the workpieces. The axial pressureis maintained until all of the energy in the rotating mass has beendissipated in the weld, thereby stopping the rotation. During the weldcycle, material that is in the interface becomes plastic as a result ofthe dissipated frictional heat, and is forged out of the weld. Theremaining plasticized material is hot worked together to accomplish theweld. The resulting loss in length of the workpieces as force is appliedand plasticized material is forced out of the contact area is referredto as an “upset”. In inertia welding tubular elements to form a lengthof tubing, both the inner and outer diameter of the resulting tube willhave flash resulting from the upset. The flash may be removed usingfinishing techniques. Because the materials joined by inertia welding donot melt during the process, no significant alloying occurs, therebyavoiding the adverse affects alloy formation has on mechanical andcorrosion properties in the weld zone.

Inertia welding may be used to join metal combinations not normallyconsidered compatible, such as, for example, aluminum to steel, copperto aluminum, titanium to copper, and nickel alloys to steel. In general,any metallic materials that are forgeable can be friction welded, suchas by inertia welding, including maraging steel, tool steel, alloysteels and tantalum. The inertia welding process is generally muchfaster than fusion welding, and the process is principally controlled bythe machine, thereby eliminating human error so that the resultant weldis independent of operator skill. There also is no need for significantweld joint preparation, and no weld wire or welding consumable isrequired.

Explosion welding is a well known solid state welding technique forjoining dissimilar materials, and the technique is generally describedthroughout the literature. Examples of such descriptions include“Explosion Welding”, Volume 6, ASM Handbook, Welding, Brazing andSoldering (ASM Intern. 1993), pages 705-718; and A. Nobili, et al.,“Recent Developments in Characterizations of Titanium-Steel ExplosionBond Interface”, 1999 Reactive Metals in Corrosive ApplicationsConference Proceedings, Sep. 12-16, 1999 (Sunriver, Oreg.) pages 89-98.In explosion welding, the controlled energy of a detonating explosive isused to create a metallurgical bond between two or more similar ordissimilar metallic materials. During the high-velocity collision of thematerials, under proper conditions a jet is formed between thematerials, which sweeps away contaminant surface films. The materials,cleaned of surface films by the jet action, are joined at an internalpoint under the influence of the very high pressure that is obtainednear the collision point. Diffusion of the materials does not occurduring explosion welding, so problematic alloys are not generated.

As used herein, “metallurgical bond” refers to a bond between matedmetallic surfaces achieved through application of pressure and/ortemperature.

In the forgoing embodiment for manufacturing urea synthesis strippertubes, for example, an explosive cladded weld joint could be formedbetween the titanium and zirconium segments of the replacement strippertube. In one embodiment of such a process, for example, zirconium andtitanium would be explosively bonded together and a small tube would bemachined from the plate. The tube would be composed of a zirconium sideand a titanium side. The zirconium would then be fusion welded to thezirconium tube portion, and the titanium would be fusion welded to thetitanium tube portion. Explosive cladded tube transition joints arecurrently produced, although the inventor is not aware of such tubeshaving a zirconium-to-titanium metal combination.

Although the foregoing specific embodiments are directed to the use ofstripper tubes within a urea synthesis unit, wherein the stripper tubesinclude a zirconium region and one or more titanium regions, it will beunderstood that the parts and methods described herein are not solimited. For example, methods according to the present disclosure may beadapted for providing original or replacement fluid conducting parts forother types of chemical processing equipment, as well as other types ofequipment, wherein one or more fluid conducting parts comprise a fluidconducting first region including a corrosion resistant materialdirectly or indirectly joined to a fluid conducting second region by asolid state welding technique so that a fluid conducting passage in thefirst region is positioned in fluid communication with a fluidconducting region of the second region. The resulting weld region doesnot suffer from significantly reduced mechanical and/or corrosionproperties relative to the first and second materials. A material withinthe second region may be selected so that it may be secured by fusionwelding to a region of the chemical processing or other equipment thatis fabricated from a compatible material. By “compatible”, it is meantthat the fusion welding process does not produce alloys in the weldregion having significantly degraded mechanical and corrosionproperties. One example is an original or replacement tube for a heatexchanger, wherein the tube is fabricated of a corrosion resistantregion and a second region as just described.

Moreover, although the above non-limiting specific embodiments includesolid state welded fluid conducting parts that have separate regionsincluding zirconium and titanium, the present method also may be appliedin cases in which the corrosion resistant first region includes one ormore zirconium alloys or other corrosion resistant materials and/orwhere the second region includes titanium alloys or other materials.Non-limiting examples of possible zirconium alloys include, for example,Zr700 (UNS R60700), Zr702 (UNS R60702), Zr705 (UNS R60705), andZircaloys (including, for example, Zr-4, Zr-2, and Zr2.5Nb). As anon-limiting example, it is contemplated that parts constructedaccording to the present disclosure may be used in equipment wherein theexisting structure to which the fluid conducting part is fused is atitanium alloy or a stainless steel, in which case the correspondingregion of the part may be fabricated from a like or substantially liketitanium alloy or stainless steel, respectively. In constructing such apart, a region including the titanium alloy or stainless steel is eitherdirectly or indirectly solid state welded to another region ofzirconium, zirconium alloy, and/or another metal or alloy providingdesired mechanical, corrosion and/or other properties.

Another possible modification of the above embodiment of the method ofthe present disclosure is to provide a multi-layer fluid conducting endor region including a corrosion resistant inner layer surrounding afluid passageway and an outer layer of another material. As used herein,“multi-layer” refers to the presence of two or more layers of differingmaterials metallurgically bonded in the referenced structure. Thecorrosion resistant material of the inner layer may be, for example,zirconium, a zirconium alloy, or another corrosion resistant metal oralloy. The multi-layer end or region may be formed by any suitablemethod, such as, for example, by co-extrusion, also known as extrusionbonding, of the layers. Co-extrusion is a method of forming tubing thatis readily familiar to those having ordinary skill, and which also isdiscussed further herein. The multi-layer fluid conducting end or regionmay be solid state welded, such as by inertia welding, to a corrosionresistant fluid conducting region formed from zirconium or anothercorrosion resistant material so that fluid conducting passages throughthe regions are in fluid communication. In this way, a highly corrosionresistant metal or alloy is provided along the entire inner length ofthe fluid conducting part. If the outer layer of the multi-layer end orregion is formed of titanium, for example, it may be fusion welded tothe titanium cladding of a stripper unit tubesheet without significantlycompromising the mechanical and corrosion properties of the material inthe vicinity of the weld.

Multi-layer tubing designs are known for nuclear cladding to containfuel pellets. The patent literature includes known methods ofmetallurgically bonding layers of zirconium-based alloys for thisparticular application. For example, a thin pure zirconium internalliner for a nuclear cladding tube is described in U.S. Pat. No.4,200,492. The zirconium liner inhibits crack initiation and propagationfrom stress corrosion cracking. A much thicker outer layer of alloyedzirconium constitutes the cladding's base material and provides suitablecorrosion resistance and mechanical properties. Additional patents suchas, for example, U.S. Pat. Nos. 5,383,228, 5,524,032 and 5,517,540,describe variations of chemistry, layer stack-up, and processing optionsfor multi-layer nuclear fuel pellet cladding. In one arrangement, a thinexternal liner has been utilized for fuel pellet cladding to improvewater-side corrosion resistance of the cladding. The present inventorsconceived of adapting certain aspects of multi-layer nuclear fuelcladding to embodiments of the fluid-conducting parts of the presentdisclosure comprising multi-layer fluid conducting part arrangements. Incontrast to certain embodiments of the present fluid-conducting parts,however, the foregoing patents are directed to nuclear fuel cladding andto bonding layers of similar zirconium-based alloys, and, for example,do not teach or suggest metallurgically bonding dissimilar reactivemetals, such as titanium and zirconium.

As noted herein, dissimilar reactive metals such as titanium andzirconium alloys are difficult to join due to, for example, differencesin their thermal expansion properties, crystal lattice size differences,and deficiencies in weld integrity when the materials are bonded.Explosion welding has been used to metallurgically bond dissimilaralloys, but this technique suffers from known shortcomings. For example,localized deformation or thinning of the bonded layers can occur due tovariation in the explosive force. As such, post-bonding machining hasbeen used, but it can be difficult to accurately control inner linerthickness during machining. Also, the pressure forces generated duringexplosion welding cause the metal to behave like a viscous fluid, whichcan lead to a wavy borderline between the bonded materials. The wavycharacter of the borderline makes difficult or impossible maintainingprecise liner thickness since the extent of the borderline can varysignificantly. In certain known explosive bonded designs, for example,the wavy borderline between the bonded materials varies from 0.5 mm to 1mm (0.0197 inch to 0.0394 inch) peak-to-peak. The geometry of parts tobe bonded also is a limiting constraint when using explosion welding. Incertain explosion welding techniques, an outer component is surroundedby explosive to implode onto an inner liner of a dissimilar materialthat has been supported with a rod to prevent collapse inward beyond apoint. In such technique the wall thickness and strength of the outercomponents are a limiting factor. In an alternative technique, explosiveis placed within the inner diameter of a liner component, and theexplosive force expands the inner liner onto the inner surface of anouter component. In such case, the inner diameter must be large enoughto contain sufficient explosive, which may preclude using the techniquein manufacturing small internal diameter, thick-walled tubes and otherfluid conducting parts, such as are used in high-pressure heatexchangers.

Several alternate methods are known for metallurgically bondingdissimilar metals and alloys. For example, U.S. Pat. No. 4,518,111provides a two-step method for bonding zirconium and steel components.In an initial step, explosion welding is used to metallurgically bondthe two components into a billet. In a second step, a steel third layeris metallurgically bonded by co-extruding the billet, thus providingthree bonded layers. Of course, use of explosion welding has thelimitations discussed above, and the use of a two-step process ofbonding the layers increases costs of the final product. U.S. Pat. No.5,259,547 also describes a two-step process including a step ofexplosion welding, followed by expanding the bonded billet over aprofiled mandrel to securely metallurgically bond the layers. Althoughmulti-layer fluid conducting parts within the present invention may beproduced using multiple-step manufacturing methods, there may be asignificant cost advantage associated with one-step bonding methods,such as those described in detail herein.

Another known approach to metallurgically bonding dissimilar metals oralloys is the use of hot isostatic pressing (HIP) to pre-bondcylindrical components before solid state bonding by extrusion. U.S.Pat. No. 6,691,397 utilizes HIPing with pressure in excess of 15,000psig and temperature over 2000° F. for at least 2 hours up to 24 hours.HIPing produces a metallurgical bond between the dissimilar metals,allowing materials of different flow stresses to maintain integrityduring hot extrusion into tube. Of course, as discussed above, atwo-step bonding process may add costs relative to a single-stepprocess. Also, initially forming a metallurgical bond between thematerials by HIPing requires significant time under pressure and attemperature. Dissimilar materials can form a brittle interdiffusionlayer at their interface, or can experience excessive grain growthduring heating for extended periods. Neither attribute is desirable ifthe extruded tube subsequently is to be cold worked.

Yet another approach to forming a metallurgical bond between dissimilarmetals or alloys is described in U.S. Pat. No. 5,558,150, in which anouter alloy layer is centrifugally cast onto an inner layer. The layersof the composite casting are metallurgically bonded upon cooling. Theprocess of this patent is designed for bonding steels and reactivemetal, which requires that casting be conducted in a vacuum to precludeoxygen and nitrogen contamination from the atmosphere. In addition, thegrain structure of cast materials is unrefined, preventing subsequentcold working.

One non-limiting embodiment of a method by which cylindricalzirconium/titanium multi-layer fluid-conducting parts or part portionsuseful in the present disclosure includes the steps generally shown inFIG. 3, as further described below.

In a first step of the method of FIG. 3, individual hollow cylindricaltitanium and zirconium components to be bonded together are provided insuitable forms, with the cylindrical zirconium liner component sized tofit within the inner diameter of the cylindrical titanium basecomponent. As an example, the base part may be Titanium Grade 3 (ASTMdesignation) and the zirconium liner part may be Zircadyne 702™ (Zr702)alloy. The surfaces of the parts to be bonded together are suitablyprepared to better ensure a satisfactory metallurgical bond between thecomponents. It is advantageous to machine, surface condition, and cleanthe surfaces to be bonded together. For example, the inventors havedetermined that when preparing titanium and zirconium prior tometallurgically bonding, it is advantageous to prepare the surfaces tobe bonded so that each has surface roughness no greater than about 63micro-inches (0.0016 mm) RA. It is believed that providing surfaces withsuch a surface finish better ensures adequate cleaning in the peaks andvalleys of the surface roughness profile. Also, it is believed that anabsence of deep grooves and scratches, for example, helps maintain acontinuous metallurgical bond between the surfaces withoutdelaminations.

It is also advantageous to clean the surfaces to be bonded of foreigncontaminants such as, for example, dirt and oil so that a high-qualitymetallurgical bond results. An example of one method that may be used toclean surfaces of reactive metals is ice blasting, which is described inU.S. Pat. No. 5,483,563. The ice blasting technique involves propellingcrystalline water against the metal or alloy surface to be cleaned,resulting in both mechanical scrubbing and liquid flushing. Ice blastingcan result in an improved integrity of the metallurgical bond betweensurfaces relative to conventional surface cleaning methods since iceblasting does not deposit a cleaning agent residue on the cleanedsurfaces. An example of such a residue is residual fluoride that may beleft behind on a surface etched with hydrofluoric-nitric acid.Non-limiting examples of alternative surface cleaning techniques includemechanical conditioning, acid etching, and use of solvent or alkalinecleaners. Those of ordinary skill will know of other suitable surfacecleaning techniques.

In a second step of the method in FIG. 3, the components are assembledso that the zirconium liner component is suitably seated within thetitanium base component, and the end joints between the components arewelded so as to provide a multi-layer billet suitable for extrusion. Aschematic end view of the multi-layer billet 110 is shown in FIG. 4,wherein 114 is the cylindrical titanium outer base material, 116 is thecylindrical zirconium inner liner, and 118 is the welded end jointbetween the base material and the liner. The weld may be, for example,an autogenous fusion weld, in which case the weld comprises atitanium-zirconium mixture. As previously described, the fusion weldingof dissimilar reactive metals produces an alloy in the weld zone thattypically has lower strength and ductility relative to the individualmetals. The integrity of the welds joining the end joints of the billet,however, is critical to prevent the atmosphere from contaminating theinterfaces of the components during preheating of the billet prior toextrusion of the billet in a succeeding step. In addition, the welds aresubjected to very large stresses during extrusion. Weld failure duringextrusion can result in atmospheric contamination or non-uniformreduction of the base and liner components during extrusion.

In one embodiment of the method of FIG. 3, an alternative technique,electron beam welding, is used to weld the end joints between the baseand liner components to provide the billet. Electron beam welding hasbeen found to provide acceptable weld penetration and weld width, and toprovide adequate protection from atmospheric contamination between theinterfaces. Preferably, the weld penetrates the end joint from 5 to 50mm (0.197 to 1.97 inch) (measured in the planes of the welded surfaces)and with a width adequate to seal the opposed surfaces of the base andliner components from the atmosphere. Suitable alternative techniques ofproviding autogenous or filler welds will be known to those havingordinary skill in the art of welding reactive metals.

In a third step of the method illustrated in FIG. 3, the billet formedin the prior step is heated and extruded to form a metallurgicallybonded, seamless tube of dissimilar metals having a substantiallyuniform liner thickness. In one embodiment of the method, thetitanium/zirconium billet is induction heated to a temperature in therange of 550° C. to 900° C. (1022° F. to 1652° F.). Alternatively, forexample, a gas or electric furnace may be used to heat the billet priorto extrusion, but such heating techniques take significantly more timeand create more surface contamination on the billet relative toinduction heating.

The heated billet is loaded into an extrusion press with suitabletooling to produce a concentric tube from the billet. In one embodimentof the method, the extrusion ram is advanced at a substantiallyconsistent 50 to 900 mm/minute (1.969 to 35.4 inches/minute) during theextrusion cycle to avoid unacceptable fluctuations in the linerthickness of the extruded tube. Factors influencing the quality of themetallurgical bond resulting from extrusion include temperature, time attemperature, pressure, and surface cleanliness. In the presentnon-limiting embodiment, for example, the extrusion ratio may range from3:1 to 30:1 to better ensure adequate pressure to metallurgically bondthe base and liner components.

A significant advantage of induction heating the billet and thenextruding the billet to metallurgically bond the layers is that the timeperiod during which the billet is heated to and held at the extrusiontemperature can be very limited. When the time at extrusion temperatureis small, little or no interdiffusion occurs between the titanium andzirconium layers when the metallurgical bond is formed during theextrusion. An interdiffusion, or simply “diffusion”, layer typicallyexists between layers of dissimilar metals that have beenmetallurgically bonded. The diffusion layer may include intermetalliccompounds or compositional gradients that are harder or more brittlethan the individual alloys. Because there is a lack of significantinterdiffusion when rapidly induction heating the billet and thenextruding the billet to metallurgically bond the layers, material thatis brittle and has high strength relative to the zirconium and titaniumlayers is not formed in significant amounts. This allows the extruded,multi-layer part to be readily cold worked, such as by, for example,cold drawing or cold tube reducing, if necessary to fabricate the finalfluid conducting part. Accordingly, one significant aspect of certainembodiments of the methods described herein is to produce a partincluding dissimilar, metallurgically bonded layers without theformation of any substantial interdiffusion layer between the bondedlayers. It can be determined that no substantial interdiffusion layerhas formed during thermal exposure from extrusion, annealing, oralternative metallurgical bonding processes if the resultingmetallurgically bonded multi-layer structure can be readily cold worked,such as by cold drawing or cold tube reducing.

In an optional fourth step of the method illustrated in FIG. 3, theextruded multi-layer tubing is heat-treated to relieve stresses withinthe material and/or recrystallize the material before application ofcold work. Preferably, the heat treatment technique minimizes thedevelopment of an interdiffusion layer between the reactivemetallurgically bonded layers. To better inhibit interdiffusion layerdevelopment, the heat treatment preferably is tailored to achievedesired stress relief and/or recrystallization in the constituentmaterials of the multi-layer tubing using the minimum necessarytemperature and time. As an example, titanium/zirconium multi-layertubing made by the present embodiment may be annealed at a temperaturein the range of 500° C. to 750° C. (932° C. to 1382° C.) for 1 to 12hours to limit the development of the interdiffusion layer. Those ofordinary skill in the heat treatment arts may readily fashion a suitableheat treatment regimen for a particular multi-layer fluid conductingpart made according to the present disclosure.

In a fifth step of the method of FIG. 3, the multi-layer tubing is coldworked. Cold working reactive metals can provide beneficial attributessuch as improved grain structure, mechanical properties, dimensions, andsurface finish. As noted above, a method of fabricating the tubing thatlimits the generation of a brittle interdiffusion layer is preferred.Possible cold working techniques useful for multi-layer tubing madeaccording to the present disclosure include, for example, cold drawing,cold tube reducing, and tube rolling with internal and external rolls,such as by flow forming. Other techniques of suitably cold working amulti-layer fluid conducting member made according to the presentdisclosure will be apparent to those of skill in the art uponconsidering the present disclosure.

Cold tube reducing (also known as “pilgering”) has been found to be aparticularly advantageous cold working technique in connection with thepresent embodiment of the method of the present disclosure. Cold tubereducing employs grooved, tapered dies that roll lengthwise over thetube while pressing the material onto a tapered mandrel. The graduallydecreasing cross-sectional area of the grooves compresses the tube wallsonto the corresponding tapered mandrel. The tube is fed longitudinallyinto the dies and is rotated about its longitudinal axis so that theentire circumference is uniformly reduced in dimension. Typicalreductions achieved when cold tube reducing tubular members of reactivemetals are in the range of 20% to 90%.

It will be understood that although the embodiment of the methodillustrated in FIG. 3 and described above utilizes a titanium basecomponent and a zirconium liner, alternative materials may be used forthe base and liner components. For example, and without intending tolimit the scope of the invention in any way, one may employ a titaniumor titanium alloy outer base and a niobium or niobium alloy inner liner,or a tantalum or tantalum alloy external liner and a titanium ortitanium alloy inner base. Other materials combinations may be selectedbased on the application for which the tubing is adapted, and suchcombinations will be apparent to those of ordinary skill uponconsideration of the present disclosure.

It also will be understood that multi-layer fluid conducting parts orpart portions made according to the present disclosure need not be madeusing the method outlined in FIG. 3. For example, alternative methodsare disclosed herein. Also, those having ordinary skill, upon readingthe present disclosure, may readily design alternate methods forproviding such multi-layer parts or part portions.

Moreover, although the present description generally refers tomulti-layer parts and part portions having two layers, more than twolayers may be provided in such parts or part portions. For example, thepart may include three or more layers, as desired, which may beassembled into a billet and processed to a fluid conducting part asgenerally described above with respect to a dual layer part. As such, itwill be understood that the scope of the present invention includesfluid conducting parts including three or more layers, including acorrosion resistant inner layer or liner surrounding a fluid conductingpassage through the part, an outer layer, and one or more intermediatelayers intermediate the inner and outer layers. In such case, the innerand outer layers are referred to herein as being “indirectly” bonded,which contrasts with the case wherein the inner and outer layers are“directly” bonded to one another. In each case, however, the immediatelyadjacent layers in the multi-layer structure are metallurgically bondedtogether. As noted, such multi-layer fluid conducting parts and partportions may be made using the teachings herein along with the knowledgeof those persons having ordinary skill.

One arrangement for securing an original or replacementzirconium/titanium stripper tube having a mono-layer tube section solidstate welded to a multi-layer tube end to a tubesheet is shown in crosssection in FIG. 5. The bi-layer tube end shown in FIG. 5 may befabricated, for example, by co-extrusion to provide an outer layer oftitanium and a corrosion resistant zirconium inner liner. With referenceto FIG. 5, stripper tube 210 includes a central cylindrical passage 212defined by tubular wall 213. A tubular zirconium region 214 is solidstate welded to a bi-layer tubular end region 216 at weld region 217.Bi-layer end region 216 includes tubular titanium outer region 219 ametallurgically bonded to tubular zirconium inner liner 219 b. Tubesheet220 includes titanium cladder sheet 224 bonded to carbon or stainlesssteel region 226. Titanium strength weld 228 is formed by fusion weldingtitanium outer region 219 a to titanium cladder sheet 224. It will beunderstood that because like materials are fusion welded to securestripper tube 210 to tubesheet 220, problematic alloys having reducedcorrosion resistance are not produced, and the mechanical properties ofthe materials in the vicinity of the weld zone are not significantlycompromised.

In a modification to what is described above, the tubes may include acorrosion resistant tubular region of zirconium, zirconium alloy, oranother corrosion resistant material, and a tubular region includingstainless steel, and the two regions are directly or indirectly joinedby inertia welding or another solid state welding technique to form aunitary tube. Stripper tubes made in this way may be used as originalequipment in newly manufactured strippers including stainless steeltubesheets, or may be used as replacement tubes to retrofit strippersincluding stainless steel tubesheets. The stainless steel of thestripper tubes is selected to be compositionally substantially identicalto the tubesheet stainless steel to which the tubes are fused. Astrength weld is formed at the junction of the tube stainless steel andthe tubesheet stainless steel to secure the tubes to the stripper unit.Of course, any of the possible materials combinations and designsdescribed herein for stripper tubes also will be useful as original orreplacement stripper tubes in particular stripper designs.

Yet another possible modification to the parts and methods describedherein is to include one or more materials intermediate regions of thepart that are joined by solid state welding. As noted, regions joinedwith such intermediate materials are referred to herein as having been“indirectly” joined by solid state welding. In the case of solid statewelding a first region of zirconium or a zirconium alloy to a secondregion of titanium or a titanium alloy, for example, possible materialsdisposed intermediate the first and second regions may include, forexample, one or more of low oxygen titanium, vanadium, tantalum,hafnium, niobium, and alloys of these materials. These intermediatematerials would be problematic if fusion welding were used, but may besuitably joined to the other materials by inertia welding.

The following examples further illustrate characteristics of embodimentsof the parts and methods described herein.

Example 1 Comparative Study of Solid State and Fusion Weld Joints

In connection with the methods disclosed herein, the mechanical andcorrosion characteristics of zirconium-to-titanium fusion weld jointswere evaluated relative to weld joints produced by solid state welding.It is well known that zirconium and titanium can be fusion welded usingtechniques such as, for example, gas tungsten arc welding, metal gas arcwelding, plasma welding, and resistance welding, to produce a highstrength weld joint. As noted above, however, the weld produced onjoining dissimilar materials by fusion welding can be affected bycorrosion and is subject to solid solution hardening that cansignificantly increase the hardness and strength of the weld zone. Inautogenous (that is, without the use of filler metal) fusion welding ofzirconium to titanium, the zirconium-titanium alloys produced in theweld zone will vary from 100% zirconium to 100% titanium. This alloyingeffect can be somewhat lessened through the use of either zirconium ortitanium filler metal. Even with the use of filler metal, a region ofthe weld will be composed of various zirconium-titanium alloycompositions, and such alloy region may have significantly compromisedcorrosion resistance and mechanical properties. The solid state weldingof tubular sections was investigated as a means to avoid melting of thejoined material during welding and creation of problematic alloys in theweld zone.

Experimental Procedure

Several weld samples were prepared by inertia welding a zirconium tubesection to a titanium tube section to create a unitary tube. FIG. 6depicts both an unsectioned inertia weld sample and a sectioned inertiaweld sample, wherein a zirconium tube section (darker colored material)has been inertia welded to a titanium tube section, creating flash onthe inner diameter and outer diameter. The flash was forced from theweld area through upset occurring during the weld cycle. Because thewelding process may cause thermal stresses in the final weld joint,certain of the inertia welded samples were stress relieved at an aim of550° C. (1022° F.) for about ½ hour to remove weld stresses. In weldedsamples where a stress relief heat treatment was used, the samples wereevaluated both before and after the heat treatment. FIG. 7 shows twofully machined inertia weld samples wherein the flash has been removed.

For purposes of comparison, several samples of a zirconium plate sectionfusion welded to a titanium plate section were prepared and evaluated.Both autogenous fusion welded samples and samples fusion welded usingfiller metal were prepared. Mechanical testing, hardness testing,metallography, scanning electron microscopy, and corrosion testing wereused to evaluate and compare the weld samples.

Mechanical and Hardness Test Results

Sub-size samples were tested at room temperature using standard tensiletesting to determine the mechanical strength of the weld joints. Tensilespecimens were machined with the center of the weld zone in the middleof the tensile gauge specimen. Specimens were tested according to ASTME-8. Table 1 provides the tensile test results for several differentsample welds. The results show that the inertia weld samples had higherultimate strength and slightly lower yield strength than the fusion weldsamples. Applying the above-described stress relief anneal to an inertiaweld sample only slightly reduced the mechanical strength of thesamples. In observing the actual tensile testing procedure, it was seenthat all of the welded samples (both inertia and fusion welded) failedin the titanium parent metal, and not in the weld areas.

TABLE 1 UTS YS Elongation Type of Weld Joint ksi (MPa) ksi (MPa) %, min.Zr/Ti Autogeneous 71.8 (495) 57.1 (394) 17 (no filler metal) 70.4 (485)55.1 (380) 12 Zr/Ti Fusion Weld 61.1 (421) 44.0 (313) 22 (Zr Filler)60.9 (420) 46.1 (318) 20 Zr/Ti Fusion Weld 70.1 (483) 53.1 (366) 16 (TiFiller) 70.6 (487) 56.1 (387) 16 Zr/Ti Inertia Weld 75.2 (519) 51.8(357) 20 (as-welded) 76.4 (527) 52.8 (364) 15 71.5 (493) 50.8 (350) 5Zr/Ti/Inertia Weld 74.6 (514) 47.8 (330) 16 (stress relieved) 74.9 (517)48.3 (333) 28 74.5 (514) 49.1 (339) 19 Wrought (non-welded)   50 (345)min.   40 (275) min. Titanium Grade 2 ASTM 20 Specification Wrought(non-welded)   65 (450) min.   55 (380) min. 18 Titanium Grade 3 ASTMSpecification Wrought (non-welded)   55 (379) min.   30 (207) min. 16Zirconium 702 ™ ASTM Specification

Table 1 also lists the ASTM requirements for titanium Grade 2, titaniumGrade 3, and Zr702. In the sample welds tested, the mechanicalproperties of each of the inertia welded tubes (stress relievedcondition) met the requirements for Zr702 grade.

Hardness of the welded samples was evaluated beginning at the zirconiumparent metal and across the weld to the titanium parent metal. Thehardness testing was conducted to determine the extent of solid solutionhardening in the zirconium to titanium fusion and inertia welds. Table 2provides the hardness test results. Given that that an alloyed weldmetal is not created during inertia welding, “N/A” is listed as thehardness of the weld metal for such samples. The results show that inthe fusion weld samples the hardness of the weld metal was more thandouble that of either parent metal. This would contribute to the fusionwelds' very poor bend ductility, and could result in premature failureof the weld. In contrast, the hardness of the heat affected zone of thetested inertia weld samples was only slightly elevated relative to theimmediately adjacent parent metal. This contrast demonstrates amechanical disadvantage resulting from the inherent creation of alloysin a fusion weld zone.

TABLE 2 Vickers Hardness (1 kg load) Tita- Zirconium Heat Heat niumParent Affected Weld Affected Parent Type of Weld Joint Metal Zone MetalZone Metal Zr/Ti Autogeneous 159 174 339 173 165 No filler Metal 158 163339 180 160 164 174 348 168 167 Zr/Ti Fusion Weld 156 165 326 165 160with Zr Filler Metal 149 163 330 176 159 153 172 339 168 161 Zr/TiFusion Weld 160 173 264 177 159 with Ti Filler Metal 163 174 279 167 154166 178 254 174 166 Zr/Ti Inertia Weld 170 217 N/A 211 184 (as-welded)173 217 N/A 199 181 175 209 N/A 197 185 Zr/Ti Inertia Weld 171 202 N/A171 161 (stress relieved) 177 200 N/A 161 171 165 206 N/A 165 170

Corrosion Test Results

The sample welds were tested for corrosion resistance in a standard Hueytest environment (65% nitric acid at a boiling temperature of 118° C.(244° F.)) according to ASTM specification A-262. The Huey test iscommonly used to evaluate corrosion resistance of materials that will beexposed to nitric acid or urea environments. There were five 48-hourtest periods, and new nitric acid was used after each test period.Nitric acid was replaced because the leaching and dissolution of Ti⁺⁴ions into the acid test solution will decrease the apparent corrosionrate of titanium in the test samples. Moreover, replacement of acidsolution better simulates the dynamic conditions occurring in equipmentsuch as heat exchangers, where acid is replenished continuously. Therate of corrosion of zirconium, however, is not affected by the presenceof either titanium or zirconium ions in the nitric acid solution.

The weld samples were subjected to the test solution for a predeterminedtime and then evaluated for weight loss using standard corrosion ratecalculations. The corrosion samples were visually and metallographicallyexamined to determine whether the weld area was preferentially attacked.Table 3 provides the corrosion test results. As shown, the corrosionrate of the fusion welded samples exceeded 15 mils/year (mpy) (0.39mm/year) for both the autogenous samples and those samples prepared withtitanium filler metal. The fusion welded samples prepared with zirconiumfiller metal showed a significantly lower 5.7 mpy (0.15 mm/year) averagecorrosion rate, but examination of the weld interface showed apreferential attack in the area near the toe of the weld.

TABLE 3 Fusion Weld Fusion Weld Autogeneous with Zirconium with TitaniumInertia Welds Weld Filler Metal Filler Metal As-Received Stress RelievedTest Period Corrosion rate mpy (mm/yr) #1 15.4 (0.39) 3.3 (0.08) 19.6(0.50) 6.3 (0.16)  6.2 (0.15)    17 (0.43) 4.7 (0.12)   35 (0.89) #216.9 (0.43) 4.7 (0.12)  7.6 (0.19) 0.6 (0.015) .4 (0.01) 19.4 (0.49) 5.7(0.15)   25 (0.63) #3 19.5 (0.49) 6.1 (0.15)  9.2 (0.23) 0 GW 22.2(0.56) 7.1 (0.18) 21.5 (0.55) #4 17.7 (0.45) 5.4 (0.14)  9.2 (0.23) 0.9(0.023) 0.8 (0.021)   18 (0.46) 6.5 (0.16) 19.7 (0.50) #5 18.4 (0.47)5.6 (0.14)  8.1 (0.21) GW 0   16 (0.41) 8.2 (0.21) 19.3 (0.49) Avg.   18(0.46) 5.7 (0.15) 17.4 (0.44) 1.6 (0.04)  1.5 (0.038) GW = gained weight

In general, the results in Table 3 show that the fusion welded sampleswould be less suitable than the inertia welded samples in hightemperature/high pressure conditions because of the fusion weldedsamples' relatively high corrosion rates. Visual examination of thefusion welded corrosion samples with autogenous welds revealed thepresence of a white corrosion film on the titanium parent metal, whichwas easily removed. A heavy white oxide also was noted on the titaniumside of the weld, which was initially easily removed but became moretenacious as the test duration increased. General corrosion was foundover the regions of the autogenous weld area where no white oxide wasfound. Visual examination of the fusion welded corrosion samples formedwith zirconium filler metal revealed that the welds apparently wereunaffected by a discolored oxide film. The titanium side was a dark graywith a thin white line at the fusion line of the weld. Heavier corrosionwas found on the fusion line on the zirconium side of the weld. Visualinspection of the fusion welded corrosion sample using titanium fillermetal revealed that the weld area was completely covered by a hard whitelayer (oxide) deposit. The titanium side of the weld deposit was gray incolor, but was lighter in color than the zirconium side. Titanium formedan easily removed light gray/white film on the samples over each of thetest periods. The calculated average corrosion rate considerablydiffered between the two test trials.

The significant difference in the corrosion test results for thezirconium to titanium fusion weld samples using zirconium filler metalrelative to the zirconium to titanium fusion weld samples using titaniumfiller metal (or the autogenous weld samples) are believed largely dueto the zirconium alloys' higher corrosion resistance relative to thetitanium alloys' resistance. Also, the zirconium filler metal coveredmost of the welded area. Therefore, the 5.7 mpy (0.15 mm/year) corrosionrate was at least in part due to the area of the toe of the weld wherethe alloying in the weld region took place.

It is difficult to evaluate erosion characteristics in the laboratory.In general, however, titanium is known to be less erosion resistant thanzirconium. As such, providing equipment with original or replacementfluid conducting parts primarily fabricated from zirconium rather thantitanium, or including a zirconium inner layer in addition to otherlayers, as according to one aspect of the present disclosure, shouldinhibit erosion. In addition, providing a co-extruded multi-layer tubeincluding an inner liner of zirconium as described above, wherein thetube end is solid state welded to a zirconium tube portion, wouldprotect the entire length of a stripper tube from both erosion andcorrosion.

Metallographic and Microscopic Examination

Metallography was used to examine the characteristics of thezirconium-to-titanium weld interfaces. FIG. 8 is a cross-section of thezirconium to titanium weld interface in the tube wall of an inertiawelded sample. The flash material is shown sweeping out of the weldjoint, but the interface between the dissimilar metals is bright anddistinct. FIG. 9 is a high magnification view of the same weldinterface. The darkened zone of each of the metals, adjacent the weldjoint, is the heat affected zone. The darkening is caused by heat inputat the bond interface and is not due to alloying. Even at highmagnification, the interface between the zirconium and titanium metalsis bright and distinct and shows no evidence of alloying.

To better characterize the weld interface of the inertia weld, scanningelectron microscopy (SEM) was used. SEM was used to better investigatewhether alloying occurred on any scale in the interface region and toassess whether any areas were present in which the two metals were nottotally bonded. FIG. 10 is a high magnification SEM image of theinterface region that was previously metallographically inspected. Noalloyed regions are apparent in the image. Energy dispersive X-rayanalysis of the interface of the same sample confirmed the absence ofalloyed regions within the inertia weld interface. Instead, the bondingregion between the two metals included a mechanical mixture, or swirl,of pure zirconium and pure titanium.

General Observations from Testing

Thus, the above test results show that the zirconium to titanium inertiawelded samples performed much better than the fusion welded samples interms of mechanical properties and corrosion resistance, and the inertiawelded samples appeared to be substantially free of alloyed regionswithin the weld zone. No obvious corrosive attack was noted in theinertia welded samples as was seen in the fusion welded samples. Thefusion welded samples had a high corrosion rate exceeding 15 mpy (0.38mm/year), while the inertia weld exhibited a corrosion rate less than 2mpy (0.05 mm/year) in testing done to evaluate corrosion resistance innitric acid and urea environments.

Example 2 Fabrication of Multi-Layer Tube

One embodiment for metallurgically bonding dissimilar reactive metalssuch as, for example, titanium and zirconium, to form a multi-layer,fluid conducting member involves three distinct process routes. Thefirst process route is directed to fabricating an outer billet, or basecomponent. The second process route is directed to fabricating an innerliner component. In the third process route, the base component and theliner component are combined into an assembled billet, and the billet isthen extruded, cold worked, and heat treated to provide the multi-layertube. In the following paragraphs, the three process routes aredescribed in greater detail as specifically applied in the production ofmulti-layer tubing including a titanium Grade 3 (UNS R50550) outer baseand a Zircadyne 702™ (Zr702) (UNS R60702) inner liner. Zr702 alloy isavailable from ATI Wah Chang, Albany, Oreg., and has the followingchemistry (in weight percentages of total alloy weight): 99.2 min.zirconium+hafnium; 4.5 max. hafnium; 0.2 max. iron+chromium; 0.005 max.hydrogen; 0.25 max. nitrogen; 0.05 max. carbon; and 0.16 max. oxygen.

Steps included in the first process route are shown schematically on theleft-hand side of FIG. 11. Titanium Grade 3 (TiGr3) was cast into aningot using conventional consumable electrode vacuum-arc meltingtechniques. The ingot was heated in the beta-phase field and forged toan intermediate diameter, followed by subsequent reductions in the alphaand alpha+beta phase fields to provide a cylindrical forging with adiameter of approximately 210 mm (8.27 inches). The forging was sawedinto individual billets. Each billet was machined to provide a hollow,cylindrical billet having approximate dimensions of 201 mm (7.91 inches)outer diameter and 108 mm (4.26 inches) inner diameter. To better ensureacceptable metallurgical bonding between the cylindrical TiGr3 billetand the zirconium inner liner, the inner diameter of the TiGr3 billetwas machined to a surface roughness of 63 micro-inches (0.0016 mm) RAmaximum. A relatively smooth surface finish better ensures adequatecleaning in the peaks and grooves of the surface roughness profile. Theabsence of significant grooves and scratches on the surface betterensures formation of a continuous metallurgical bond between base andliner components that does not suffer from delaminations.

Steps in the second process route are shown schematically on theright-hand side of FIG. 11. This route relates to fabricating the Zr702alloy inner liner of the multi-layer tube. Zr702 alloy was cast into aningot and forged in a manner similar to the TiGr3 alloy above. The linerwas machined from a 115 mm (4.53 inches) diameter cylindrical forging.(In one non-limiting alternative arrangement, liners may be formed byextruding an oversize tube and then sawing individual liners forsubsequent machining.) The machined Zr702 alloy liner was approximately108 mm (4.26 inches) outer diameter×54 mm (2.13 inches) inner diameter,with an outer diameter surface roughness of 63 micro-inches (0.0016 mm)RA maximum. The outer diameter surface roughness was maintained withinsuch limits for the purposes mentioned above with respect to the surfaceroughness of the TiGr3 cylindrical billet inner diameter. The liner wasmatch machined with precise tolerances to slip within the TiGr3 billet.A preferred tolerance for the gap between the inner diameter of the baseand the outer diameter of the liner is about 0.25 mm (about 0.010 inch).

In the third process route, shown schematically in FIG. 12, the TiGr3outer component and the Zr702 alloy liner component were assembled intoa billet and then metallurgically bonded and reduced into smallerdiameter multi-layer tubing. Before assembly, the outer component andliner component were cleaned by ice blasting to remove foreigncontamination, such as dirt and oil. Clean surfaces are important so asto from a high-quality metallurgical bond.

The cleaned and dried billet and liner components were slipped togetherinto an assembled billet. The billet's end joints were welded in avacuum of at least 1×10⁻³ torr (0.133 Pa) using an electron beam gun.The electron beam was focused on the end joints to produce a weld with apenetration of 10 to 40 mm into the billet and with a weld width of atleast 5 mm. Weld integrity is important to preventing the atmospherefrom contaminating and inhibiting formation of the metallurgical bondduring extrusion of the assembled billet. FIG. 13 schematically shows anend view of the welded assembled billet 310, wherein 312 is the TiGr3outer base component, 314 is the Zr702 alloy inner liner component, 316is the weld zone including a mixture including titanium and zirconium,and 318 is the cylindrical fluid conducting void passing through thebillet.

Any weld splatter was ground off the welded assembled billet. The billetwas then induction heated in a cylindrical coil to 650-775° C.(1202-1427° F.), with an aim of 700° C. (1292° C.) and transferred to a3500 ton Lombard hydraulic extrusion press. The billet was placed in acylindrical container with a mandrel inserted into the inner diameter ofthe liner components to establish the extruded inner diameter size. Astem on the extrusion press pushed the billet through a conical dieusing upset pressure of about 1500 tons (8.896×10³ N) to extrude thebillet into a seamless multi-layer tube. The extrusion elongation ratiowas approximately 11:1, and the aim was to provide an extruded tubehaving 3.100 inches (78.74 mm) outer diameter and a wall thickness ofabout 0.525 inch (13.4 mm). The dissimilar metals interacted and weremetallurgically bonded upon extrusion as a result of conditionsincluding process temperature, time-at-temperature, pressure, andcleanliness of the mating surfaces. Several inches of the lead end andtail end of the metallurgically bonded multi-layer extrusion wereremoved by sawing to ensure uniform liner thickness in the remainingportion.

The extruded tube was pickled in HF/nitric acid for a time sufficient toremove 0.001-0.002 inch (0.0254-0.508 mm) per wall. The tube was thencold worked on a pilger mill to further reduce tube diameter and wallthickness. In the pilger mill, the tube was rolled lengthwise bygrooved, tapered dies that pressed the material over a similarly taperedmandrel. The tube was fed into the dies and rotated around itslongitudinal axis to substantially uniformly reduce the entirecircumference of the tube during each stroke of the mill. Themulti-layer tube was reduced using a first pass on the pilger mill to anintermediate size of 44.5 mm (1.75 inches) outer diameter and 6.3 mm(0.25 inch) wall thickness. The rocked tube was cleaned using analkaline cleaner, water rinse, and a pickle in 70% nitric acid, and thenheat treated by vacuum anneal to recrystallize and soften the material.The heat treatment included annealing the tube at a temperature of621±28° C. (1150±50° F.) for 1-2 hours. Other possible anneal regimensinclude heating at other temperatures in the range 500° C. (932° F.) to750° C. (1382° F.) for 1-12 hours. The heat treatment should be adaptedto minimize growth of intermetallic particles or compositional gradientsthat are harder and more brittle than the base and liner alloys. Abrittle and/or wide diffusion zone can lead to delaminations of the tubelayers.

Subsequent to annealing, the tube was pickled in 70% nitric acid toremove any vacuum anneal stain, and then rotary straightened. The tubewas then re-heated and subjected to a second pilger pass to reduce thetube to final dimensions of 27.0 mm (1.06 inch) outer diameter and 3.5mm (0.138 mm) wall thickness. The final zirconium liner thickness wasapproximately 0.9 mm (0.035 inch). FIG. 14 is a micrograph of themetallurgical bond region of one of the multi-layer tubes made by theprocess. The image shows a fine grain structure (which should providesubstantially uniform mechanical properties) and a continuousmetallurgical bond between the titanium and zirconium layers. Themetallurgical bond prevents the type of crevice corrosion seen in theknown mechanically bonded (snug fit) tube designs.

The mechanical strength of TiGr3/Zr702 alloy multi-layer tubes madeusing the process in this example were evaluated and compared withproperties of a TiGr3 mono-tube. The properties of 27.0 mm outerdiameter×3.5 mm inner diameter samples of each tube type are shown inTable 4 below. The mechanical properties are similar, which shows thatthe Zircadyne® 702 liner does not significantly degrade the evaluatedmechanical properties of the TiGr3 base material.

TABLE 4 UTS YS Elongation Tube Type Sample ksi (MPa) ksi (MPa) (%, min.)T-Gr3/Zircadyne 702 ™ 1 77.9 (537) 59.5 (410) 32 Multi-layer Tube 2 81.6(562) 59.6 (411) 35 TiGr3 Mono-Tube 1 80.1 (552) 63.3 (436) 37 2   81(558) 61.1 (421) 35

Portions of a tube formed by the method described in the present examplecan be solid state welded to the ends of a length of fluid conductingtube composed of zirconium or some other corrosion resistant metal oralloy to form a composite tube suitable for use in retrofitting thestripper of urea synthesis equipment. In such case, as described above,the material of the multi-layer tube's outer layer may be selected sothat fusion welding the outer layer to the tubesheet will not result insignificant reduction in the corrosion resistance of the weld region.For example, the TiGr3/Zr702 alloy multi-layer tube made in the presentexample would be particularly advantageous for use in retrofitting astripper including a titanium-clad tubesheet.

Multi-layer tubes and other fluid conducting parts made by the presentexample also may be used without being solid state welded to amono-layer fluid conducting part. In certain of such embodiments, thematerial of the outer layer of the multi-layer tube or other part may beselected so that when that material is fusion welded to a tubesheet orother mounting portion of the equipment, no problematic alloys areproduced that would substantially negatively impact on the corrosionresistance, mechanical, or other important properties of the tube/partor mounting portion.

Of course, it will be understood that although the present discussionhas focused on use of the multi-layer tubing formed in the presentexample in a stripper apparatus, the tubing also may be used as a fluidconducting part in other apparatus, including, for example, otherchemical process equipment, heat exchangers, and any other equipmentnoted herein.

It is to be understood that the present description illustrates thoseaspects relevant to a clear understanding of the disclosure. Certainaspects that would be apparent to those skilled in the art and that,therefore, would not facilitate a better understanding have not beenpresented in order to simplify the present disclosure. Although thepresent disclosure has been described in connection with certainembodiments, those of ordinary skill in the art will, upon consideringthe foregoing disclosure, recognize that many modifications andvariations may be employed. It is intended that the foregoingdescription and the following claims cover all such variations andmodifications.

What is claimed is:
 1. A method for making a tube, the methodcomprising: providing a hollow first cylinder comprising zirconium or azirconium alloy, the first cylinder having an outer surface; providing ahollow second cylinder comprising titanium or a titanium alloy, thesecond cylinder having an inner surface, wherein the first cylinder canfit within the second cylinder; positioning the first cylinder withinthe second cylinder, wherein the outer surface of the first cylinderopposes the inner surface of the second cylinder; welding together endjoints between the first cylinder and the second cylinder to provide amulti-layer billet; induction heating the multi-layer billet to anextrusion temperature; extruding the heated multi-layer billet, therebymetallurgically bonding the outer surface of the first cylinder to theinner surface of the second cylinder, and providing a tube having a wallcomprising an inner layer comprising zirconium or a zirconium alloymetallurgically bonded to an outer layer comprising titanium or atitanium alloy; and cold working the tube to reduce the wall thicknessand/or diameter of the tube.
 2. The method of claim 1, wherein the tubelacks an interdiffusion layer between the inner layer and the outerlayer.
 3. The method of claim 1, wherein the tube lacks intermetalliccompounds and alloying in the metallurgical bond between the inner layerand the outer layer.
 4. The method of claim 1, wherein cold working thetube comprises cold pilgering the tube.
 5. The method of claim 1,wherein cold working the tube comprises cold pilgering the tube to areduction in area of 20% to 90%.
 6. The method of claim 1, wherein coldworking the tube comprises cold drawing the tube.
 7. The method of claim1, further comprising heat treating the tube after extruding themulti-layer billet and/or after cold working the tube.
 8. The method ofclaim 7, wherein heat treating the tube comprises annealing the tube ata temperature in the range of 500° C. to 750° C. for 1 to 12 hours. 9.The method of claim 1, wherein cold working the tube comprises at leasttwo cold pilgering passes, wherein the tube is annealed at a temperaturein the range of 500° C. to 750° C. for 1 to 12 hours after eachpilgering pass and before any subsequent pilgering pass.
 10. The methodof claim 1, comprising extruding the billet in an extrusion apparatuscomprising an extrusion ram that is advanced at a rate in the range of50 mm/minute to 900 mm/minute during an extrusion cycle.
 11. The methodof claim 1, comprising extruding the billet at an extrusion ratio in therange of 3:1 to 30:1.
 12. The method of claim 1, wherein the multi-layerbillet is induction heated to a temperature in the range of 550° C. to900° C.
 13. The method of claim 1, wherein the end joints are weldedtogether with an electron beam weld.
 14. The method of claim 13, whereinthe electron beam weld penetrates the end joints to a depth ranging from5 mm to 50 mm.
 15. The method of claim 1, further comprising reducingsurface roughness of at least one of the outer surface of the firstcylinder and the inner surface of the second cylinder to no greater than63 micro-inches RA before positioning the first cylinder within thesecond cylinder.
 16. The method of claim 1, further comprising iceblasting at least one of the outer surface of the first cylinder and theinner surface the second cylinder by propelling crystalline wateragainst the surface(s), thereby mechanically scrubbing and liquidflushing the surface(s).
 17. The method of claim 1, further comprisinginertia welding a zirconium or zirconium alloy tube segment to the tubehaving a wall comprising an inner layer comprising zirconium or azirconium alloy metallurgically bonded to an outer layer comprisingtitanium or a titanium alloy.
 18. A method for making a tube, the methodcomprising: providing a hollow first cylinder comprising zirconium, azirconium alloy, hafnium, a hafnium alloy, vanadium, a vanadium alloy,niobium, a niobium alloy, tantalum, or a tantalum alloy, the firstcylinder having an outer surface; providing a hollow second cylindercomprising stainless steel, titanium, or a titanium alloy, the secondcylinder having an inner surface, wherein the first cylinder can fitwithin the second cylinder; positioning the first cylinder within thesecond cylinder, wherein the outer surface of the first cylinder opposesthe inner surface of the second cylinder; welding together end jointsbetween the first cylinder and the second cylinder to provide amulti-layer billet; heating the multi-layer billet to an extrusiontemperature; extruding the heated multi-layer billet, therebymetallurgically bonding the outer surface of the first cylinder to theinner surface of the second cylinder, and providing a tube having a wallcomprising an inner layer metallurgically bonded to an outer layer; andcold working the tube to reduce the wall thickness and/or diameter ofthe tube.
 19. The method of claim 18, wherein cold working the tubecomprises cold pilgering the tube to a reduction in area of 20% to 90%.20. The method of claim 18, further comprising annealing the tube at atemperature in the range of 500° C. to 750° C. for 1 to 12 hours,wherein the annealing is performed after extruding the multi-layerbillet and/or after cold working the tube.
 21. The method of claim 18,comprising extruding the billet at an extrusion ratio in the range of3:1 to 30:1 in an extrusion apparatus comprising an extrusion ram thatis advanced at a rate in the range of 50 mm/minute to 900 mm/minuteduring an extrusion cycle.
 22. The method of claim 18, wherein themulti-layer billet is induction heated to a temperature in the range of550° C. to 900° C.
 23. The method of claim 18, wherein the end jointsare welded together with an electron beam weld that penetrates the endjoints to a depth ranging from 5 mm to 50 mm.
 24. The method of claim18, further comprising reducing surface roughness of at least one of theouter surface of the first cylinder and the inner surface of the secondcylinder to no greater than 63 micro-inches RA before positioning thefirst cylinder within the second cylinder.
 25. The method of claim 18,further comprising ice blasting at least one of the outer surface of thefirst cylinder and the inner surface the second cylinder by propellingcrystalline water against the surface(s), thereby mechanically scrubbingand liquid flushing the surface(s).
 26. A method for replacing a tube inchemical processing equipment, the method comprising: providing a tubemade in accordance with the method of claim 1; and fusion welding thetube to a tubesheet in the chemical processing equipment.
 27. The methodof claim 26, wherein the chemical processing equipment comprises a ureastripper, a heat exchanger, or a condenser.